In general, in an internal combustion engine having a plurality of cylinders, cylinder-to-cylinder variations in intake air quantity (quantity of air introduced into a cylinder) may result due to the difference in shapes of intake manifolds of the cylinders or variations in valve clearances among intake valves. Such cylinder-to-cylinder variations in the intake air quantity may cause cylinder-to-cylinder variations in torque or in air-fuel ratio. When the cylinder-to-cylinder variations in torque increase, variations in engine torque in a cycle increase, whereby vibrations may cause uncomfortable feeling to a driver. When the cylinder-to-cylinder variations in air-fuel ratio increase, variations in air-fuel ratio of exhaust gas, which flow into a catalyst in a cycle, increase correspondingly. Consequently, the margin of fluctuations of air-fuel ratio of exhaust gas may exceed the purification capacity of the catalyst, whereby the rate of exhaust gas purification may be lowered.
As a countermeasure of such problems, some methods for correcting cylinder-to-cylinder variations in torque or variations in air-fuel ratio are proposed. For example, Patent Document 1 (JP-A-62-17342), proposes a technology in which a torque sensor provided on a crankshaft detects torque generated at each cylinder, and the fuel injection quantity of each cylinder is corrected, so that average torque of all the cylinders is produced in each cylinder.
Patent Document 2 (JP-A-2000-220489), proposes a technology in which the air-fuel ratio of each cylinder is estimated based on the output from an air-fuel ratio sensor provided on an exhaust pipe, and the fuel injection quantity of each cylinder is corrected so as to reduce the cylinder-to-cylinder variations in air-fuel ratio.
In general, a throttle valve controls the intake air quantity. In recent years, however, a variable intake valve mechanism is provided for varying the lift amount of the intake valve. The lift amount of the intake valve is controlled according to the position of an accelerator, or the conditions of the engine operation, so that the intake air quantity is controlled. Such intake air quantity controlled by the variable intake valve is advantageous, because the intake air quantity may be reduced by reducing the lift amount of the intake valve, without throttling the air intake passage by the throttle valve. Thus pumping loss may be reduced, thereby reducing fuel consumption.
However, in the intake air quantity control with the variable intake valve, since the lift amount of the intake valve decreases at low load, the actual lift amount with respect to the rate of the target lift amount varies increasingly from cylinder to cylinder (variations due to cylinder-to-cylinder differences in component tolerance and assembling tolerance), which may lead to increase in cylinder-to-cylinder variations in intake air quantity. Therefore, fluctuations in torque or the air-fuel ratio in each cylinder may result due to the effect of cylinder-to-cylinder variations in intake air quantity, and hence cylinder-to-cylinder variations in torque or in air-fuel ratio may increase.
In the above Patent Documents 1 and 2, toque and the air-fuel ratio are detected for every individual cylinder, and cylinder-to-cylinder variations in torque and in air-fuel ratio are corrected by manipulating the fuel injection quantity of each cylinder based on the detected data. However, when cylinder-to-cylinder variations in intake air quantity increase, it becomes difficult to correct cylinder-to-cylinder variations in torque and in air-fuel ratio with high degree of accuracy simply by correcting the fuel injection quantity. In addition, it is also difficult to correct such variations with high degree of accuracy when the cylinder-to-cylinder variations in torque and in air-fuel ratio is generated from a combination of a plurality of causes such as cylinder-to-cylinder variations in intake air quantity and in intake fuel quantity.
Furthermore, an airflow meter or an intake pipe pressure sensor is mounted to an intake pipe assembly, which tends to be affected by the reflected waves of the intake air pulsation or the air intake interference of other cylinders. Therefore, the output waveforms from the airflow meter or the intake pipe pressure sensor contains noise caused by the reflected waves of the intake air pulsation or air intake interference of other cylinders. Therefore, the output waveforms from the airflow meter or the intake pipe pressure sensor do not become pulsation waveforms which reflect cylinder-to-cylinder variations in intake air quantity with high degree of accuracy in some operating ranges due to the effect of reflected waves or the intake air interference. Thus cylinder-to-cylinder variations in intake air quantity may not be detected with high degree of accuracy.
When a vehicle is actually traveling, since the operating condition changes every second, so that time (or the number of times) for sampling the outputs from the airflow meter or from the intake pipe pressure sensor cannot be secured sufficiently in some operating ranges, which may also become a cause of failure to detect the cylinder-to-cylinder variations in intake air quantity with high degree of accuracy.